Enhancing Photovoltage of Metal-Insulator-Semiconductor (MIS) Photoanode By Coordination Polymer Via Facile Chemical Deposition

Abstract

Photoelectrochemical (PEC) water splitting is a promising route to convert intermittent sunlight into chemical fuels, which can be stored and used on-demand to produce electricity. One of the remaining challenges for the development of efficient PEC water splitting systems is related to the photoanode, which requires a high-performance light absorber with an excellent catalyst to drive the OER that needs high potential [1]. The metal-insulator-semiconductor (MIS) architecture employing Si as the photoanode is an attractive system due to the wide light absorption range of Si (Eg 1.1 eV) and high OER catalytic activity of metal [2-3]. For the latter, metal nanoparticles including Co and Ni can be dispersed on the semiconductor surface via electrodeposition in high-performance Si MIS photoanodes [2]. Furthermore, surrounding the metal nanoparticles and the uncoated Si with a higher-work-function material leads to significant improvement of photovoltage due to the pinch-off effect [4]. Up to this point, the surrounding layer is generally the oxidized phase of the base metal, such as its oxide or oxyhydroxide, and the fabrication techniques used to introduce the surrounding layer are limited to a few choices, i.e., electrodeposition, photodeposition, or annealing methods [4]. Copper(I) thiocyanate (CuSCN) is a coordination polymer which can be prepared by a simple chemical deposition through conversion of Cu [6]. Because Cu and CuSCN have work functions of 4.5-4.9 [7] and 5.4 eV [8], respectively, the Cu/CuSCN junction on n-Si is expected to develop the pinch-off region and increase the performance of Si/SiOx/Cu photoanode. In this presentation, we aim to highlight two key developments of Cu-based MIS Si photoanodes prepared by facile methods. Firstly, we employed electrodeposition to coat 30-nm Cu nanoparticles on n-Si, resulting in a photoanode with a low OER onset potential of 1.37 V vs reversible hydrogen electrode (RHE). Secondly, we added CuSCN on the n-Si/SiOx/Cu photoanode via a simple chemical deposition in an aqueous bath containing ammonium thiocyanate (NH4SCN) and ethylenediaminetetraacetic (EDTA). The presence of CuSCN on n-Si/SiOx/Cu was confirmed by X-ray photoelectron spectroscopy (XPS), scanning electron microscopy (SEM) and Raman spectroscopy. The optimal n-Si/SiOx/Cu/CuSCN with a short CuSCN deposition time showed a 100-mV improvement in the onset potential relative to the unmodified n-Si/SiOx/Cu. Based on open-circuit-potential (OCP) measurements, the CuSCN-modified photoanode exhibited a high photovoltage of 460 mV. We believe that the findings can provide a new strategy for further surface modification of PEC devices to achieve efficient and low-cost solar harvesting technology.[1] Walter, M. G.; Warren, E. L.; McKone, J. R.; Boettcher, S. W.; Mi, Q.; Santori, E. A.; Lewis, N. S. Solar Water Splitting Cells. Chem. Rev. 2010, 110 (11), 6446–6473.[2] Loget, G. Water Oxidation with Inhomogeneous Metal-Silicon Interfaces. Curr. Opin. Colloid Interface Sci. 2019, 39 (l), 40–50.[3] Aroonratsameruang, P.; Pattanasattayavong, P.; Dorcet, V.; Mériadec, C.; Ababou-Girard, S.; Fryars, S.; Loget, G. Structure–Property Relationships in Redox-Derivatized Metal–Insulator–Semiconductor (MIS) Photoanodes. J. Phys. Chem. C 2020, 124 (47), 25907–25916.[4] Laskowski, F. A. L.; Oener, S. Z.; Nellist, M. R.; Gordon, A. M.; Bain, D. C.; Fehrs, J. L.; Boettcher, S. W. Nanoscale Semiconductor/Catalyst Interfaces in Photoelectrochemistry. Nat. Mater. 2020, 19 (1), 69–76.[5] Lee, S. A.; Choi, S.; Kim, C.; Yang, J. W.; Kim, S. Y.; Jang, H. W. Si-Based Water Oxidation Photoanodes Conjugated with Earth-Abundant Transition Metal-Based Catalysts. ACS Mater. Lett. 2020, 2 (1), 107–126.[6] Xu, J.; Xue, D. Fabrication of Upended Taper-Shaped Cuprous Thiocyanate Arrays on a Copper Surface at Room Temperature. J. Phys. Chem. B 2006, 110 (23), 11232–11236.[7] Gartland, P. O.; Berge, S.; Slagsvold, B. J. Photoelectric Work Function of a Copper Single Crystal for the (100), (110), (111), and (112) Faces. Phys. Rev. Lett. 1972, 28 (12), 738–739.[8] Worakajit, P.; Hamada, F.; Sahu, D.; Kidkhunthod, P.; Sudyoadsuk, T.; Promarak, V.; Harding, D. J.; Packwood, D. M.; Saeki, A.; Pattanasattayavong, P. Elucidating the Coordination of Diethyl Sulfide Molecules in Copper(I) Thiocyanate (CuSCN) Thin Films and Improving Hole Transport by Antisolvent Treatment. Adv. Funct. Mater. 2020, 30 (36), 2002355.